The index profile of an optical fiber is generally described in terms of the appearance of the graph plotting the refractive index of the fiber as a function of radius. In conventional manner, the distance r to the center of the fiber is plotted along the abscissa axis and the difference between the refractive index and the refractive index of the fiber cladding is plotted up the ordinate axis. The index profile can thus be said to be “stepped”, “trapezium-shaped”, or “triangular” for graphs that are respectively step-, trapezium-, or triangle-shaped. Such curves are generally idealized profiles or reference profiles for the fiber, and fiber fabrication constraints can lead to a profile that departs perceptibly therefrom.
It is conventional for the line fiber in optical fiber transmission systems to be a step index fiber, also referred to as a single mode fiber (SMF). The Applicant company thus sells a single mode step index fiber under the reference ASMF 200 which presents a chromatic dispersion canceling wavelength λ0 in the range 1300 nanometers (nm) to 1320 nm, and chromatic dispersion that is less than or equal to 3.5 picoseconds per nanometer kilometer (ps/(nm.km)) in the range 1285 nm to 1330 nm, and of 17 ps/(nm.km) at 1550 nm. At 1550 nm the chromatic dispersion slope is about 0.06 ps/(nm2.km).
WO-A-00 36443 describes a step index optical fiber presenting a core of index greater than that of the cladding. The cladding is surrounded in a layer of carbon. The core presents a diameter lying in the range 9.5 micrometers (μm) to 12.0 μm. The relative difference in index between the core and the cladding lies in the range 0.3% to 0.5%. In that document, it is stated that the fiber can be fabricated using silica, by doping the core with germanium and using silica cladding; an alternative is to dope the cladding with fluorine, while using a silica core.
Both of those solutions suffer from drawbacks. Firstly, doping the core with germanium requires germanium to be used at high concentration—typically greater than 5% by weight—in order to achieve the required index difference relative to silica cladding. Such germanium concentrations increase attenuation in the fiber. Furthermore, fabricating a silica-core fiber with doped cladding, as suggested in that document, implies using a vapor axial deposition (VAD) technique during fabrication, or else using an outside vapor phase oxidation (OVPO) technique. WO-A-00 42458 describes a transmission fiber for long-distance transmission systems; the cladding is fluorine-doped and the core is chlorine-doped. The fiber is fabricated using a VAD technique.
To manufacture optical fibers, the modified chemical vapor deposition (MCVD) technique is also used. Layers of silica containing dopant for varying its index are deposited successively inside a deposition tube. Thereafter the tube is collapsed or contracted so as to constitute a first preform. This first preform is inserted in one or more sleeves which are collapsed or contracted in turn so as to press against the first preform. The resulting preform is drawn to form a fiber. Such techniques for fabricating a fiber are well known to the person skilled in the art.
Thus, EP-A-0 972 752 describes MCVD fabrication and it proposes depositing successive layers of cladding material and of core material inside a deposition tube. After contraction, the deposition tube is inserted in one or more sleeves; it is proposed that the inner sleeve should present doping to lower its index so as to constitute a buried-cladding fiber. The cladding deposited inside the deposition tube can be doped with fluorine, the core being doped with germanium. Providing the purity of the deposition tube is sufficient, it is possible to avoid depositing cladding. The deposition tube is a glass tube in which the concentration of OH− ions is less than 0.05 parts per million (ppm) by weight. U.S. Pat. No. 4,566,754 or U.S. Pat. No. 5,692,087 thus propose a step index fiber manufactured by MCVD, in which fluorine-doped cladding and a germanium-doped core are deposited inside a silica deposition tube.
For the same type of preform, U.S. Pat. No. 5,942,296 suggests facilitating drawing down the preform by acting on the viscosity and the thermal conductivity of the silica deposition tube and of the sleeve(s) surrounding it. That solution makes it possible to avoid heating the core of the preform. It is specified in that document that the cost of MCVD fabrication decreases with decreasing thickness of the fluorine-doped cladding.
EP-A-0 899 243 also proposes a step index fiber presenting a germanium-doped core, inner cladding that is fluorine-doped, and outer cladding of non-doped silica. That application proposes drawing the fiber at a speed greater than 20 grams per minute (g/min).
EP-A-0 863 108 describes a method of fabricating a preform by plasma deposition of build-out material on the outside of the deposition tube.